These are still expensive in comparison to this ElectricIMP module >> http://electricimp.com/ a complete single module consisting of Cortex-M3 and WiFi at about 25 USD, How they are able to get this price?

Check ebay or other online shops for the nrf24l01 - should be about $2-3 smiley

The Nordic is a great module but it is not a completely functional plug and play module in that format. It requires peripheral components including some sort of PCB, adapter board, or socket to hold that dinky 4mm chip.

The biggest expense associated with building these types of functional modules is the PCB. If you total all of the components needed at wholesale price its probably about $5. So yeah its economics of scale as mentioned above. If you build ten of them you are going to pay a lot more than if you build 100,000 of them. This becomes really clear if you have looked into having PCB's manufactured either raw or populated.

Correct, the nRF24L01 is a wireless transceiver that uses the 2.4GHz band. Same frequency as Wifi but that doesn't make it a Wifi device. I have a setup at home were an ATmega328 with some sensors connected to it sends an array of data to an ATmega1284 through 2 nRF24L01 modules. That ATmega1284 sends data over I2C to an Arduino Uno with an Ethernet shield. The Uno writes that data to an sd card and runs a small webserver. Sounds complicated, but it wasn't very difficult to program. And I'm very new to micro controllers and programming.